Presently, modems are used to transport digital data between data terminal equipment such as personal computers, workstations and the like, over channels, such as telephone lines. Typically, the communication signal includes digital data in the form of a carrier signal that is modulated by a predetermined transmit constellation of signal points. Each signal point of the transmit constellation represents a digital code or value to be transported. At the sending modem, a carrier signal is thus modulated at the constellation signal point corresponding to the digital code or value to be transported over the channel. At the receiving modem, the modulated carrier signal may include distortion and other impairments induced as a result of the transmission process. The carrier signal is removed, and a received signal point is then identified from a receive constellation. The digital code or value represented by the signal point may then be obtained from a memory device, such as a look-up table.
The present application is directed to modems employing echo cancellation and may be used, for example, with a full duplex modem. Briefly, a modem with echo cancellation, in accordance with the present invention, is especially applicable for modems having echo cancellers in which the echoes to be cancelled have a substantial phase jitter component.
Echo cancellers enable modems to operate in a full-duplex mode over two-wire lines at higher data rates than would otherwise be practical without echo cancellation. As used herein, the term "echoes" refers to undesirable reflections of a locally-transmitted signal. These reflections are primarily due to impedance discontinuities in the communication channel. For example, such discontinuities often result at an interface between the line and other equipment. Multiple echoes may result when multiple discontinuities are present such as at a local telephone central office, a remote telephone central office, or at the remote line termination. Also, it is well-known that the use of a satellite link in a communication system provides a substantial time delay, and hence echoes, with substantial time delays.
A "near-end" echo refers to an echo having a relatively small time delay, which typically results due to the nearest line discontinuity, such as at the local telephone central office. A "far-end" echo is an echo with a substantially longer time delay, which results from remote line discontinuities. Because the near-end echo has travelled a relatively short distance, it is not common for the near-end echo to have substantial phase jitter.
Near-end and far-end echo cancellers are discussed, for example, in Dennis B. McMahan, "Modem with Improved Remote Echo Location and Cancellation," U.S. Pat. No. 4,970,715, which patent issued on Nov. 13, 1990, which patent is assigned to Universal Data Systems, Inc., the same assignee as in the present application, and which patent is hereby incorporated by reference verbatim, with the same effect as though the same patent were fully and completely set forth herein.
As used herein, "phase jitter" of an echo refers to an echo having a varying phase relative to the phase of the corresponding transmitted signal. One common cause of phase jitter is the coupling of 60 Hertz ("Hz") alternating current ("AC") signal and its harmonics to telephone lines carrying the modem signal. Thus, the 60 Hz AC signal (or its harmonics) modulate the communications signals. Far-end echoes, which must travel substantial distances, thus have a greater chance of including undesired phase jitter.
As is known, the receiver of a modem determines what digital data was transmitted based on the received signal. A receiver may also include an equalizer which attempts to compensate for non-linear amplitude versus frequency characteristics of the communications channel. It is also common for the receiver to include a phase-locked loop ("PLL") to track the carrier frequency of the received signal. A receiver including a frequency-adaptive phase jitter canceller is discussed, for example, in Michael D. Turner et al., "Frequency Adaptive Phase Jitter Canceler," U.S. Pat. No. 4,777,640, which patent issued on Oct. 11, 1988, which patent is assigned to Universal Data Systems, Inc., the same assignee as in the present application, and which patent is hereby incorporated by reference verbatim, with the same effect as though the same patent were fully and completely set forth herein.
A modem echo canceller operates on received or incoming signals and provides an output signal to the receiver in which echoes have been minimized. The echo cancellation function is very important as failure to substantially cancel incoming echoes will result in the receiver having to function with undesired echo signals present in the received signal.
Conventional echo cancellers develop an echo replica signal which corresponds to the average phase and amplitude of the received echoes. The replica signal is then subtracted from the incoming signal which contains desired signals and echoes, thereby cancelling the echoes. Because phase jitter creates varying phases about the average phase of the echo, conventional echo cancellers have not been able to generate a true echo replica which includes the phase jitter variations.
The problem of phase jitter in echo cancellation was recognized, for example, in Gitlin et al., U.S. Pat. No. 4,072,830, dated Feb. 7, 1978 ("Gitlin"), at column 1, lines 11-34. Gitlin is typical of a number of prior approaches to echo cancellation which include, as an integral part of the echo cancelling mechanism, a second order phase locked loop. These systems are capable of removing both constant and time-varying phase shifts introduced by the echo path, up to the limits of the algorithm for ensuring loop stability. See, Gitlin, column 6, lines 8-16. In practice, these systems can typically cancel frequency offset of as much as 10 Hz and remove phase jitter in the frequency range of 1 to about 10 Hz. However, phase jitter is typically caused as a result of cross talk from power lines or from telephone company ring generators, which tend to produce phase jitter in the 50-60 Hz region and in the 15-25 Hz region, respectively. Second and third harmonics often accompany these primary frequencies, as well.
In summary, prior modem echo cancellers have been limited in their capacity to address phase jitter, especially when channels having substantial phase jitter are encountered.
Thus there exists a need for a modem with improved echo cancellation which particularly addresses phase jitter.